Method and apparatus for locating mobile receivers using a wide area reference network for propagating ephemeris

ABSTRACT

An apparatus for distribution and delivery of global positioning system (GPS) satellite telemetry data using a communication link between a central site and a mobile GPS receiver. The central site is coupled to a network of reference satellite receivers that send telemetry data from all satellites to the central site. The mobile GPS receiver uses the delivered telemetry data to aid its acquisition of the GPS satellite signal. The availability of the satellite telemetry data enhances the mobile receiver&#39;s signal reception sensitivity.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of co-pending U.S. patentapplication Ser. No. 09/989,625, filed Nov. 20, 2001, which is adivisional of U.S. Pat. No. 6,411,892, issued Jun. 25, 2002, each ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates to signal processing in GPSreceivers. In particular, the present invention relates to a method andapparatus for delivering satellite data to GPS receivers to enable a GPSreceiver to acquire and lock on to GPS satellite signals in low signalstrength environments (e.g., indoors).

[0004] 2. Description of the Background Art

[0005] Conventional GPS receivers require an inordinate amount of timeto acquire and lock onto the satellite signals. Then, once locked, a GPSreceiver extracts telemetry data (almanac and ephemeris) from thesignal. From these data the GPS receiver can calculate information thatenhances its ability to lock onto the satellite signal. A relativelyhigh signal strength satellite signal is necessary to enable the systemto achieve an initial lock. Once the GPS signal is acquired, the signalstrength must remain high while the almanac and/or ephemeris data isextracted from the satellite signal. Any severe attenuation of thesignal can cause a loss of lock and the signal will requirere-acquisition. As such, the system has an inherent circularity thatmakes it difficult or impossible for GPS receivers to acquire signals inlow signal strength environments.

[0006] To aid initial acquisition of the satellite signal, many GPSreceivers store a copy of the almanac data, from which the expectedDoppler frequency of the satellite signal can be calculated. Severaltechniques have been developed to calculate useful information at aseparate GPS receiver and then transmit this data to another GPSreceiver. U.S. Pat. No. 6,064,336, issued May 16, 2000, collects almanacdata at a separate GPS receiver, then transmits the almanac data to amobile receiver. The mobile receiver then uses the almanac data tocompute the expected Doppler frequency of the satellite signal, thusaiding in initial signal acquisition.

[0007] The advantage of receiving the almanac is that each GPS satelliterepeatedly transmits a complete almanac containing orbit information forthe complete GPS constellation, thus a single GPS receiver, tracking anysatellite, can collect and propagate the almanac for all satellites inthe constellation. The disadvantage of using the almanac is that it is afairly rough model of the satellite orbit and satellite clock errors,thus the almanac is only useful in reducing the frequency uncertaintyand cannot be used to enhance receiver sensitivity by reducing thesearch window of code-delay uncertainties.

[0008] If a GPS receiver had a complete set of ephemeris data for allsatellites in view, before said receiver attempted to lock onto thosesatellites, the receiver would have significantly improved acquisitiontimes and enhanced sensitivity. This is because the ephemeris datacontains an accurate description of the satellite position, velocity,and clock errors; and the GPS receiver can use this data to increase itssensitivity by reducing significantly the search windows for frequencyuncertainty and code-delay uncertainty. The disadvantage of theephemeris is that each satellite only transmits its own ephemeris; thusa single GPS receiver cannot collect and propagate ephemeris for all thesatellites in the constellation.

[0009] Therefore there is a need in the art for a GPS receiver systemthat propagates satellite ephemeris for all satellites in theconstellation, thereby enhancing the speed of acquisition and signalsensitivity of mobile receivers.

SUMMARY OF THE INVENTION

[0010] The invention comprises a method and apparatus for distributionand delivery of the Global Positioning System (GPS) satellite ephemerisusing a communication link between a central site and a wide areanetwork of GPS receivers. The wide area network of GPS receiverscollects the ephemeris data that is transmitted by the satellites andcommunicates the data to the central site. The central site delivers theephemeris to the mobile receiver. The mobile GPS receiver uses thedelivered data to enhance its sensitivity in two ways. First, the dataallows the receiver to detect very weak signals that the receiver wouldnot ordinarily be able to detect, and second, the GPS receiver does nothave to track the satellite signals for very long before a position canbe calculated.

[0011] In one embodiment of the invention, the satellite ephemeris datais retransmitted without manipulating the data in any way. The GPSreceiver may then use this data exactly as if the receiver had receivedthe data from the satellite. In another embodiment, a satellitepseudo-range model is computed at the central site from the ephemerisdata, and this pseudo-range model is transmitted to the GPS receiver.The pseudo-range model has the characteristic that the model is moreconcise than the complete ephemeris. As such, the GPS receiver does nothave to perform as many calculations when using the pseudo-range modelas when using the complete ephemeris.

BRIEF DESCRIPTION OF DRAWINGS

[0012] The teachings of the present invention may be readily understoodby considering the following detailed description in conjunction withthe accompanying drawings, in which:

[0013]FIG. 1 depicts an architecture for a wide area reference stationnetwork in accordance with the present invention;

[0014]FIG. 2 depicts a GPS orbital sphere;

[0015]FIG. 3 depicts the intersection of the GPS orbital sphere and thehorizon planes of three reference stations;

[0016]FIGS. 4A and 4B depict the intersection of the GPS orbital sphereand the horizon planes of four reference stations;

[0017]FIG. 5 depicts a flow diagram of a method of generatingpseudo-range models;

[0018]FIG. 6 is a graph illustrating the timing (pseudo-range) andfrequency (pseudo-range rate) uncertainty for a mobile GPS receiver, andthe improvement in sensitivity that is gained by reducing both theseuncertainties;

[0019]FIG. 7 depicts a flow diagram of a method of searching through thetime (pseudo-range) and frequency (pseudo-range rate) windows; and

[0020]FIG. 8 depicts a flow diagram of a method for using pseudo-rangeinformation of satellites having high signal strength to improvereceiver sensitivity for signals received from satellites having lowsignal strength.

DETAILED DESCRIPTION OF THE INVENTION

[0021] To facilitate understanding, the description has been organizedas follows:

[0022] Overview, introduces each of the components of the invention, anddescribes their relationship to one another.

[0023] Global Tracking Network, describes how a worldwide network oftracking stations is constructed and deployed to ensure that allsatellites are tracked at all times.

[0024] Ephemeris Processing, describes an embodiment of the inventionthat provides a more compact, and simpler, model of the satelliteephemeris.

[0025] Signal Detection, describes how the retransmitted satelliteephemeris data is used in a GPS receiver to detect signals that wouldotherwise be undetectable.

[0026] Sensitivity Enhancement, describes how the two strongestsatellite signals may be used to compute the time and correlator offsetat the mobile receiver. This information is, in turn, used to enhancesensitivity for weaker GPS signals that are received by the mobilereceiver.

[0027] Overview

[0028]FIG. 1 depicts one embodiment of a global positioning system (GPS)satellite data distribution system 100 comprising:

[0029] a) A reference station network 102 comprising a plurality oftracking stations 104 ₁, 104 ₂, . . . 104 _(n) coupled to one anotherthrough a communications network 105. The reference stations 104 aredeployed over a wide area and contain GPS receivers 126 so thatephemeris may be collected from all satellites 106 within a globalnetwork of satellites e.g., the global positioning system (GPS).Ephemeris information comprises a 900 bit packet containing satelliteposition and clock information.

[0030] b) A central processing site 108 that collects the ephemeris fromthe tracking stations 104 comprises an ephemeris processor 128 thatremoves duplicate occurrences of the same ephemeris, and provides thelatest ephemeris data for redistribution to mobile GPS receivers 114 and118.

[0031] c) A communications link 120 from the central processing site tothe mobile GPS receiver 114. The link 120 may be a landline 110, orother direct communications path, that couples the mobile GPS receiver114 directly to the central processing site 108. Alternatively, thislink may have several parts, for example: a landline 112 to a wirelesstransmitter 116, and a wireless link 122 from the transmitter 116 to amobile receiver 118.

[0032] d) A mobile GPS receiver 114 or 118 that uses the redistributedephemeris data (or a modified form thereof) to aid the receiver indetecting GPS signals from satellites 106 in a satellite constellation.

[0033] e) A position processor 130, where the position of a GPS receiver114 or 118 is calculated. This could be the GPS receiver itself, thecentral processing site 108, or some other site to which the mobile GPSreceivers send the measurement data that has been obtained from thesatellites 106.

[0034] In operation, each of the satellites 106 continually broadcastephemeris information associated with a particular satellite. Tocomprehensively and simultaneously capture the ephemeris data of all thesatellites 106 in the constellation, the network 106 is spreadworldwide.

[0035] To obtain all the ephemeris data, three or more tracking stations104 are needed. Each of the 28 satellites has an orbit inclined at 55degrees relative to the equator of the earth. As such, no satellite evertravels outside of a plus or minus 55 degree range on an orbital sphere.Consequently, three stations placed 120 degrees apart and lying exactlyon the equator of the earth, would have all the satellites in view.However, placing reference stations at or close to those exact locationson the equator is impractical. To place reference stations in largecities around the world, a realistic, minimum number that will achieveviewing of all the satellites 106 is four.

[0036] Each of the tracking stations 104 contains a GPS receiver 126that acquires and tracks satellite signals from all satellites 106 thatare in view. The stations 104 extract the ephemeris information thatuniquely identifies the position of each satellite as well as satelliteclock information e.g., a 900 bit packet with a GPS signal. Theephemeris information is coupled to the central processing site 108 via,for example, a terrestrial land line network 105.

[0037] The central processing site 108 sends all or part of theephemeris information to one or more mobile GPS receivers 114 and 118.If the central processing site knows the approximate position of themobile GPS receiver, the central processing site 108 may only send theephemeris information for satellites that are presently (or about to be)in view of the mobile GPS receiver 114 or 118. The ephemeris informationcan be coupled directly through a land line 110 or other communicationpath (e.g., internet, telephone, fiber optic cable, and the like).Alternatively, the ephemeris information can be coupled to a mobile GPSreceiver 118 through a wireless system 116 such as a cell phone,wireless Internet, radio, television, and the like. The processing andutilization of the ephemeris information is described below (seeEPHEMERIS PROCESSING and SIGNAL DETECTION).

[0038] Global Tracking Network

[0039] The global GPS reference network 102 has stations 104 arrangedsuch that all satellites are in view all the time by the trackingstations 104 in the network 102. As such, the ephemeris for eachsatellite 106 is available to the network in real time, so that thenetwork, in turn, can make the ephemeris, or derived pseudo-rangemodels, available to any mobile receiver that needs them.

[0040] The minimum complete network of reference stations comprisesthree stations, approximately equally placed around the earth, on orclose to the equator. FIG. 2 shows the GPS orbital sphere 202surrounding the earth 204, and an indication 206 of all orbits of thesatellites. FIG. 3 shows the intersection of the horizon planes of 3tracking stations, (denoted A, B, and C), with the GPS orbital sphere.In FIG. 3, the orbital sphere is shaded gray in any region above thehorizon of a tracking station. Regions on the orbital sphere above thehorizons of two tracking stations are shaded slightly darker. Theorbital sphere is white in the regions, above and below 55 degrees,where there are no GPS satellites. From FIG. 3, it is clear that anypoint on any GPS orbit is always above the horizon of at least onereference station A, B or C.

[0041] It is not commercially or technically practical to placereference stations around the equator. Preferred sites are major citieswith good communications infrastructure to enable the ephemeris to becoupled to the control processing site via a reliable network. When thereference stations are moved away from the equator, more than threestations are needed to provide coverage of all satellites all the time.However, it is possible to create a network of only four referencestations with complete coverage of all GPS satellites all the time, andwith the four stations located in or near major cities. For example, thestations may be placed in Honolulu, Hi. (USA), Buenos Aires (Argentina),Tel Aviv (Israel) and Perth (Australia). FIGS. 4A and 4B show theintersection of the horizon planes of these stations with the GPSorbital sphere. Any point of any GPS orbit is always above the horizonof at least one of the reference stations. FIGS. 4A and 4B show theorbital sphere viewed from two points in space, one point (FIG. 4A) inspace approximately above Spain, and the other (FIG. 4b) from theopposite side of the sphere, approximately above New Zealand. The figureis shaded in a similar way to FIG. 3. Gray shading shows regions of theGPS orbital sphere above the horizon of at least one tracking stationand darker gray regions represent portions of the orbital sphereaccessible to two stations.

[0042] Ephemeris Processing

[0043] The ephemeris is used to compute a model of the satellitepseudo-range and pseudo-range rate. From the pseudo-range rate themobile GPS receiver can calculate the Doppler frequency offset for thesatellite signal. The computation of the pseudo-range model can be doneat the mobile receiver, or at the central processing site. In thepreferred embodiment the pseudo-range model is computed at the centralsite as follows.

[0044]FIG. 5 depicts a flow diagram of a method 500 for generating apseudo-range model. At step 502, the ephemeris data from all thetracking stations is brought to the central processing site. Ephemerisdata is transmitted continually by all satellites, mostly this isrepeated data; new ephemeris is typically transmitted every 2 hours. Theephemeris is tagged with a “Time of Ephemeris”, denoted TOE. This tagindicates the time at which the ephemeris is valid. Ephemeriscalculations are highly accurate within 2 hours of TOE. A satellitefirst transmits an ephemeris 2 hours ahead of the TOE, thus anyephemeris is highly accurate for a maximum of four hours.

[0045] At step 506, the central processing site keeps all the ephemerisdata with TOE closest to the time T at which the mobile receiverrequires ephemeris (or a pseudo-range model). Time T is provided by themobile receiver at step 504. Usually T will be the current real time,however, it could be a time up to 4 hours in the future for mobilereceivers that are collecting ephemeris/pseudo-range models in advanceof when they need them. T could also be a time in the past, for mobilereceivers processing previously stored data.

[0046] At step 508, the central processing site then calculates thesatellite positions at time T. In the preferred embodiment, this isperformed using the equations provided in the GPS Interface ControlDocument, ICD-GPS-200-B.

[0047] At step 512, the central processing site obtains the approximateposition of the mobile GPS Receiver. In the preferred embodiment, themobile GPS receiver communicates with the central processing site over awireless communications link, such as a 2-way paging network, or amobile telephone network, or similar 2-way radio networks. Such 2-wayradio networks have communication towers that receive signals over aregion of a few miles. The central processing site obtains the referenceID of the radio tower used to receive the most recent communication fromthe mobile GPS. The central processing site then obtains the position ofthis radio tower from a database. This position is used as theapproximate mobile GPS position.

[0048] In an alternative embodiment, the approximate position of themobile GPS receiver may be simply the center of the region served by aparticular communications network used to implement this invention.

[0049] In another alternate embodiment, the approximate position of themobile GPS receiver may be the last known point of said receiver,maintained in a database at the central processing site.

[0050] It is understood that many combinations and variants of the abovemethods may be used to approximate the mobile GPS receiver position.

[0051] Having calculated the satellite positions, and obtained theapproximate user position, the central processing site computes (at step510) which satellites are, or will soon be, above the horizon at themobile GPS receiver. For applications requiring simply theredistribution of the ephemeris data, at step 514, the centralprocessing site now outputs the ephemeris for those satellites above, orabout to rise above, the horizon.

[0052] In the preferred embodiment, a pseudo-range model is computedthat comprises: time T, and, for each satellite above, or about to riseabove, the horizon: the satellite PRN number, pseudo-range, pseudo-rangerate, and pseudo-range acceleration.

[0053] To compute a pseudo-range model, the central processing sitefirst computes at step 516 the pseudo-ranges of all satellites above, orabout to rise above, the mobile GPS receiver horizon. The pseudo-rangeis the geometric range between the satellite and the approximate GPSposition, plus the satellite clock offset described in the ephemeris.

[0054] At step 518, the pseudo-range rate may be computed from thesatellite velocity and clock drift. Satellite velocity may be obtaineddirectly by differentiating the satellite position equations (inICD-GPS-200-B) with respect to time. In an alternative embodiment,satellite velocity may be computed indirectly by computing satellitepositions at two different times, and then differencing the positions.

[0055] In another alternative embodiment, the pseudo-range rates may becomputed indirectly by computing the pseudo-ranges at two differenttimes, and then differencing these pseudo-ranges.

[0056] At step 520, the pseudo-range acceleration is then computed in asimilar fashion (by differentiating satellite velocity and clock driftwith respect to time, or by differencing pseudo-range rates).

[0057] The complete pseudo-range model is then packed into a structureand output to the mobile GPS receiver at step 522.

[0058] The mobile GPS receiver may use the pseudo-range model for theperiod of validity of the ephemeris from which it was derived. To applythe pseudo-range model at some time after time T, the mobile receiverpropagates the pseudo-ranges and range rates forward using the rate andacceleration information contained in the pseudo-range model.

[0059] In an alternative embodiment, the central processing sitepropagates the unaltered ephemeris 519 and the derivation of thepseudo-range model and pseudo-range rate is performed at the mobile GPSreceiver.

[0060] Krasner (U.S. Pat. No. 6,064,336) has taught that theavailability of Doppler information can aid the mobile GPS receiver byreducing the frequency uncertainty. U.S. Pat. No. 6,064,336 describes asystem and method for delivering to a mobile receiver Almanacinformation from which Doppler may be derived; or for deliveringequivalent information, derived from the Almanac; or for delivering theDoppler measurement itself from a base station near to the mobilereceiver. In another alternative embodiment of the current invention,the Ephemeris may be used to derive Doppler information. In the sectionthat follows (SIGNAL DETECTION) it will be appreciated that the use ofthis Doppler information will aid in signal acquisition to the extent ofreducing the Pseudo-range rate uncertainty, i.e., the number offrequency bins to search, but the Doppler information will not reducethe Pseudo-range uncertainty (i.e. the code delays).

[0061] Signal Detection

[0062] There are several ways in which the availability of ephemerisdata (or the derived pseudo-range model) aid the signal acquisition andsensitivity of the mobile GPS receiver, described below.

[0063] The ephemeris or pseudo-range models can predict the elevationangle to the satellite, allowing the receiver to focus on acquiring highelevation satellite signals, which are generally less subject toobstruction. Satellites that are calculated to be below the horizon(negative elevation angles) can be ignored. This satellite selection canalso be performed using an almanac of satellite orbital information, butproviding models, or ephemeris from which models can be created,eliminates the need for non-volatile storage for the almanac within themobile receiver. Thus, the ephemeris provides some advantage in thisrespect, however the main advantage of the invention is in theimprovement in signal acquisition and receiver sensitivity, describedbelow.

[0064] The “re-transmitted” or “re-broadcast” ephemeris informationimproves the operation of the mobile receiver in two ways.

[0065] First, the mobile receiver does not need to collect the ephemerisfrom the satellite. The ephemeris is broadcast from a satellite every 30seconds and requires 18 seconds to transmit. In order to receiveephemeris without the use of the present invention, a mobile receiverneeds clear, unobstructed satellite reception for the entire 18-secondinterval during which the ephemeris is being transmitted. Depending onthe environment and usage of the receiver, it may be minutes before thesituation allows the ephemeris to be collected and in many applications,for example, indoor use, the mobile receiver may never have anunobstructed view of a satellite. To eliminate the data collectiondelay, the present invention provides the ephemeris data directly to themobile receiver.

[0066] Second, the ephemeris is used, as described above, to form thepseudo-range models of the satellite signals being received at themobile receiver. These models can accelerate the acquisition process inseveral ways.

[0067] The models predict the pseudo-range and pseudo-range rate of thereceived signals. If the approximate user position is fairly accurate,these models will be very accurate in estimating the pseudo-range andpseudo-range rate. Using the models, the receiver can focus thecorrelation process around an expected signal.

[0068]FIG. 6 shows a graph 601 that illustrates the usual frequency andtiming uncertainty for a mobile GPS receiver. On the y-axis 602, thevarious rows show different pseudo range rates, and on the x-axis 604the various columns show different pseudo ranges. Without an accuratemodel, such as available using the present invention, the possibilitiesfor range rates will vary considerably because a wide range of satellitemotions are possible, and the possibilities for ranges will also varyover many cycles of the PN codes. With an accurate model provided by theephemeris information, the uncertainties can be reduced to a smallrange, depicted by the black cell 606. Many receivers will be able tosearch this small range in a single pass that eliminates a timeconsuming sequential search and allows the use of longer integrationtimes for better sensitivity, as will now be described.

[0069] Better sensitivity is achieved as follows: The sensitivity of aGPS receiver is a function of the amount of time that the receiver canintegrate the correlator outputs. The relationship between sensitivityand integration time is shown by the graph 608. With many bins tosearch, the integration time 610 equals the total available search timedivided by the number of search bins. With only a single bin to search,the integration time 612 equals the total available search time,increasing the sensitivity as shown 608.

[0070] It should be noted that in some receivers, the pseudo-ranges andpseudo-range rates that can be predicted from the pseudo-range modelswill not be accurate because of a lack of synchronization of the localclock. In this case, a search over a wide range of uncertainties willstill be initially required, but only for the strongest satellite(s). Ifthe local clock is known to be correct to within approximately onesecond of GPS time then any one satellite will be enough to synchronizethe local correlator offset. Thereafter, the expected pseudo-range andpseudo-range rates can be accurately computed for the remainingsatellites. If the local clock is not known to within approximately onesecond, then two satellites must be used to compute the two requiredclock parameters: the local clock and the correlator offset. The factthat two satellites are required is an often misunderstood point. In theGPS literature, it is often mentioned that one satellite is enough tosolve for an unknown clock offset without realizing that this is onlytrue for systems where the local clock is already approximatelysynchronized with GPS time. In traditional GPS receivers thatcontinuously track the GPS signals, the local clock is synchronized toGPS time to much better than one second accuracy. In some more modernimplementations (e.g., U.S. Pat. No. 6,064,336), the local clock issynchronized to a network time reference, which is synchronized to GPStime. However, the current invention is specifically intended to operatein implementations where the local clock is not synchronized to GPStime. The manner in which one solves for these clock parameters isdescribed in detail below.

[0071] Once the unknown clock parameters have been computed, theparameters can then be used to adjust the pseudo-range models for theremaining, weaker satellites to reduce the range of uncertainty back toa narrow region; thus enhancing sensitivity precisely when highsensitivity is needed, i.e., for detecting the weaker satellite signals.

[0072] In other receivers, the local clock and clock rate may be quiteaccurate. For example, if the clock signals are derived from a wirelessmedia that is synchronized to GPS timing (e.g., a two-way pagingnetwork), then the clock parameters are typically accurate. In thiscase, there is no clock effect and a narrow search region can be usedfrom the onset.

[0073] To quantify the benefits of the invention, consider an examplewhere the user position is known to within the radius of reception of a2-way pager tower (2-miles). In this case the pseudo-range (expressed inmilliseconds) can be pre-calculated to an accuracy of one-hundredth of amillisecond. Without the invention, a GPS receiver would search over afull millisecond of all possible code delays to lock onto the codetransmitted by the satellite. Using the invention the search window isreduced by up to one hundred times, making the GPS receiver faster, and,more importantly, allowing the use of longer integration times (asdescribed above), making the receiver capable of detecting weakersignals, such as occur indoors.

[0074] An additional advantage of having ephemeris, or the derivedpseudo-range model, at the mobile receiver is that the process ofidentifying the true correlation is more robust, since, apart fromincreasing the integration time as described above, the chance that a“false peak” would be identified is greatly reduced by considering onlycorrelations that occur within the expected range.

[0075] One embodiment of the use of ephemeris (or derived pseudo-rangemodels) to enhance sensitivity is described further with respect to FIG.7.

[0076]FIG. 7 is a flow diagram of a method 700 of signal search. Themethod begins at step 702 with an input of the pseudo-range model. Asnoted earlier this pseudo-range model is calculated from the ephemeris,either at the mobile receiver itself, or at the central processing site.At step 704, the model is applied at the current time in the mobiledevice and is used to estimate the expected current frequency and timingof GPS satellite signals, as well as the expected uncertainties of thesequantities, to form a frequency and code delay search window for eachsatellite. This window is centered on the best estimates of frequencyand delay but allows for actual variations from the best estimates dueto errors in the modeling process including inaccuracies in the roughuser position, errors in the time and frequency transfer from thewireless carrier etc. In addition, the frequency uncertainty is dividedinto a number of frequency search bins to cover the frequency searchwindow. As shown in FIG. 6, the number of search bins is dramaticallyreduced by using the pseudo-range model.

[0077] In step 706, the detection and measurement process is set toprogram the carrier correction to the first search frequency. At step708, a code correlator is invoked to search for signal correlationswithin the delay range of the delay window. Such a code correlator isstandard in the art, but the present invention dramatically reduces thenumber of possible code delays over which the correlator must searchthereby increasing the integration time for each code delay, and thusthe sensitivity of the receiver.

[0078] At step 710, the method 700 queries whether a signal is detected.If no signal is detected, the carrier correction is set, at step 712, tothe next search frequency and the search continues until a signal isfound or the frequency search bins are exhausted.

[0079] If, at step 710, the method 700 affirmatively answers the query,the signal is used at step 714 to further improve the estimate of clocktime delay and clock frequency offset. This information is utilized atstep 716 to re-compute the frequency and delay search windows for theremaining undetected satellites. In step 718, the process continuesuntil all satellites have been detected or the search windows have beenexhausted.

[0080] The method of FIG. 7 is illustrative of one of a variety ofalgorithms that can be used to guide the search process based on the GPSsignal processing's ability to estimate time and frequency.Additionally, the algorithms could be altered to include various retrymechanisms since the signals themselves may be fading or blocked.

[0081] Sensitivity Enhancement

[0082] To enhance the sensitivity of the receiver (as described withrespect to FIG. 6), the invention uses the approximate position of themobile device to compute expected pseudo-range, this reduces thepseudo-range uncertainty. However, before the inventive receiver cancompute the expected pseudo-range the following three items arerequired:

[0083] 1. the approximate position of the mobile device (to within a fewmiles of a true position)

[0084] 2. the approximate time at the mobile device (to withinapproximately one second of the true time)

[0085] 3. the correlator clock offset at the mobile device (to within afew microseconds of the true offset).

[0086] The more accurately each of the three terms is known, the moreprecisely the invention can bound the pseudo-range uncertainty, and thusthe greater the sensitivity (see FIG. 6). In the preferred embodiment,the approximate position of the mobile device is determined from theknown location of the radio tower last used by the device. The radius ofreception of radio towers for 2-way pagers and cell-phones is typically3 kilometers. Thus the approximate position of the mobile device isknown to within 3 kilometers, and the induced error on the pseudo-rangeestimate will be no more than 3 kilometers. With reference to FIG. 6.,note that the full pseudo-range uncertainty for an unaided GPS receiveris equal to one code epoch, which is approximately 300 kilometers. Thus,even knowing an approximate position as roughly as 3 kilometers canreduce the pseudo-range uncertainty one hundred times.

[0087] The timing errors also induce errors on the expectedpseudo-range. To compute expected pseudo-range, the receiver mustcalculate the satellite position in space. The satellite range from anylocation on earth varies at a rate between plus and minus 800 meters persecond. Thus each second of time error will induce a range error (andpseudo-range error) of up to 800 meters.

[0088] The mobile device correlator delay offset induces a direct errorin the pseudo-range measurement, as is well known in the GPS literature.Each microsecond of unknown correlator delay offset induces 300 metersof error in the range measurement.

[0089] Thus, to keep the pseudo-range estimate within a range of a fewkilometers (as illustrated in FIG. 6), the receiver of the presentinvention requires estimates of position, time and correlator delayoffset in the ranges shown above.

[0090] In an implementation where the real time at the mobile device isnot known to better than a few seconds, and the correlator delay offsetis not known, one solves for both using two satellite measurements, asfollows.

[0091] The equation relating pseudo-range errors to the two clock errorsis:

y=c*dt _(c)−rangeRate*dt _(s)  (1)

[0092] where:

[0093] y is the “pseudo-range residual”, i.e., the difference betweenthe expected pseudo-range and the measured pseudo-range;

[0094] c is the speed of light;

[0095] dt_(c) is the correlator delay offset; and

[0096] dt_(s) is the offset of the real time estimate.

[0097]FIG. 8 depicts a flow diagram of a method 800 for improving theclock parameters, and then improving the receiver sensitivity. Method800 comprises:

[0098] Step 802. Using the best known clock parameters, compute expectedpseudo-ranges for all the satellites.

[0099] Step 804. Measure the pseudo-ranges for the two strongestsatellites with the highest signal strength.

[0100] Step 806. Using these two measurements, solve equation (1) forthe two unknowns: dt_(c) and dt_(s).

[0101] Step 808. Use dt_(c) and dt_(s) to improve the estimate of theexpected pseudo-ranges for the remaining (weaker) satellites.

[0102] Step 810. Use these improved expected pseudo-ranges to reduce thepseudo-range uncertainty, thus improving the sensitivity of thereceiver, as shown in FIG. 6.

[0103] Although various embodiments which incorporate the teachings ofthe present invention have been shown and described in detail herein,those skilled in the art can readily devise many other variedembodiments that still incorporate these teachings.

What is claimed is:
 1. A method for locating position comprising:receiving satellite telemetry data from all of the satellites in aglobal positioning system constellation of satellites using a network oftracking stations, where the network includes enough tracking stationsto observe only a portion of an orbital sphere for the globalpositioning system constellation, the portion selected to include all ofthe orbits of the satellites; communicating the received satellitetelemetry data to a central processing site; propagating selectedsatellite telemetry data to a mobile receiver; and acquiring at leastone satellite signal at said mobile receiver using said selectedsatellite telemetry data.
 2. The method of claim 1 wherein the portionof the orbital sphere observed by the tracking stations is between plusand minus 55 degrees relative to the equator of the earth.
 3. The methodof claim 1 wherein the selected satellite telemetry data comprises theephemeris data for each satellite in view of the mobile receiver.
 4. Themethod of claim 3 wherein the selected satellite data comprises apseudo-range model, derived from the ephemeris data, that represents arelative position of each satellite in view of the mobile receiver. 5.The method of claim 3 wherein the selected satellite telemetry datacomprises a Doppler measurement derived from the satellite ephemerisdata.
 6. The method of claim 1 wherein said acquiring step furthercomprises: using the selected satellite telemetry data to narrow afrequency uncertainty and a code uncertainty.
 7. The method of claim 1further comprising: computing a position of said mobile receiver usingsaid selected satellite data.
 8. The method of claim 7 wherein saidcomputing step is performed within the mobile receiver.
 9. The method ofclaim 7 wherein said computing step is performed at a location that isremote from said mobile receiver.
 10. The method of claim 1 wherein saidat least on satellite signal is a signal having a high signal strengthand said acquiring step further comprises: using the at least oneacquired satellite signal to aid in receiving other satellite signalshaving low signal strength.
 11. The method of claim 10 wherein said atleast one acquired satellite signal is used to generate a clock and acorrelator delay offset.
 12. The method of claim 10 wherein said atleast one acquired satellite signal is used to improve an estimatedpseudo-range computation for satellite signals having low signalstrength.
 13. Apparatus for locating a position of a mobile receivercomprising: a network of satellite signal receivers for receivingsatellite signals from all satellites in a constellation of globalpositioning satellites, where the network includes enough satellitesignal receivers to observe only a portion of an orbital sphere for theconstellation of global positioning satellites, the portion selected toinclude all of the orbits of the satellites; a communications network,coupled to each of said satellite signal receivers in said plurality ofsatellite signal receivers; a satellite data processor, coupled to saidcommunications network; and a mobile receiver, coupled to said satellitedata processor.
 14. The apparatus of claim 13 wherein the portion of theorbital sphere observed by the satellite signal receivers is betweenplus and minus 55 degrees relative to the equator of the earth.
 15. Theapparatus of claim 13 further comprising a wireless network forcommunicating said satellite data to said mobile receiver.
 16. Theapparatus of claim 13 wherein said satellite data processor generates apseudo-range model for each mobile receiver and communicates thepseudo-range model to the mobile receiver.
 17. The apparatus of claim 13wherein said satellite constellation is a global positioning system(GPS) satellite constellation.
 18. Apparatus for providing satellitedata to a mobile receiver comprising: a network of tracking stations forreceiving telemetry data from satellites in a constellation, where thenetwork includes enough tracking stations to observe only a portion ofan orbital sphere for the constellation, the portion selected to includeall of the orbits of the satellites; and a communication network forpropagating the telemetry data from all the satellites to a dataprocessor.
 19. The apparatus of claim 18 wherein the portion of theorbital sphere observed by the tracking stations is between plus andminus 55 degrees relative to the equator of the earth.
 20. The apparatusof claim 18 wherein said data processor transmits said data to a mobilereceiver.
 21. The apparatus of claim 18 wherein said data processorproduces a pseudo-range model using said telemetry data.
 22. A methodfor locating position comprising: receiving satellite telemetry datafrom all of the satellites in a global positioning system constellationof satellites using a plurality of tracking stations, where theplurality of tracking stations are positioned to observe only a portionof an orbital sphere for the global positioning system constellation,the portion selected to include all of the orbits of the satellites;communicating the received satellite telemetry data to a centralprocessing site; propagating selected satellite telemetry data to amobile receiver; and acquiring at least one satellite signal at saidmobile receiver using said selected satellite telemetry data.
 23. Themethod of claim 22 wherein the portion of the orbital sphere observed bythe tracking stations is between plus and minus 55 degrees relative tothe equator of the earth.